It is known from such as U.S pat. No. 6,431,802 (or W00025976, EP1123175, CA2349579 and DE19850708) that prior art spatial two rotational and one translational parallel mechanism comprises a holding bracket, a movable platform and three branch joint assemblies of the same structure. The branch joint assemblies each consist of five components, i.e., a driving device, a linear guide, a carriage, a hinge and a connecting rod. The parallel mechanism is structurally symmetrical about an axis in space. An end-effector is mounted on the movable platform. The moving direction of the guide is parallel to the symmetrical axis and the carriage is attached to the linear guide via a sliding joint. The connecting rod has its one end connected with the movable platform by a hinge having three rotational degrees of freedom, and has its the other end connected with the carriage through a hinge with a rotational degree of freedom. Both the driving device and linear guide are fixedly disposed on the holding bracket, whereas the carriage is driven by a servo-motor via a ball screw-nut or another linear driving device. In the parallel mechanism utilizing this type of construction, when driven by the servo-motor via the ball screw-nut, each carriage can move independently along the linear guide and accordingly, translation of the movable platform along the symmetrical axis with respect to the holding bracket, as well as rotation thereof about two orthogonal axes perpendicular to the axis are obtained.
In the aforementioned mechanism, as the linear guide is fixed on the holding bracket, a drawback arises: due to the large size of the holding bracket in the moving direction of the guide, high inertia load will be generated during high speed movement of the holding bracket along with the other moving components connected therewith.